1. Technical Field
The present disclosure relates to acoustic devices, particularly, to an ultrasonic acoustic device.
2. Description of Related Art
Acoustic devices generally include a signal device and a speaker. Signals are transmitted from the signal device to the speaker. The speaker converts the electrical signals into sound. There are different types of speakers that can be categorized according to their working principle, such as electro-dynamic loudspeakers, electromagnetic loudspeakers, electrostatic loudspeakers, and piezoelectric loudspeakers. However, the various types ultimately use mechanical vibration to produce sound waves, in other words they all achieve “electro-mechanical-acoustic” conversion.
In a paper entitled “The Thermophone” by Edward C. WENTE, Phy. Rev, 1922, Vol.XIX, No.4, p333-345, and another paper entitled “On Some Thermal Effects of Electric Currents” by William Henry Preece, Proc. R. Soc. London, 1879-1880, Vol.30, p408-411, a thermoacoustic effect was proposed. Sound waves based on the thermoacoustic effect are generated by inputting an alternating current to a metal foil, wherein or metal foil acts as a thermoacoustic element. The thermoacoustic element has a low heat capacity and is thin, so that it can transmit heat to surrounding gas medium rapidly. When the alternating current passes through the thermoacoustic element, oscillating temperature is produced in the thermoacoustic element according to the alternating current. Heat wave excited by the alternating current is transmitted in the surrounding gas medium, and causes thermal expansions and contractions of the surrounding gas medium, and thus, a sound pressure is produced.
In another article, entitled “The thermophone as a precision source of sound” by H. D. Arnold and I. B. Crandall, Phys. Rev. 10, pp22-38 (1917), a thermophone based on the thermoacoustic effect is disclosed. Referring to FIG. 12, a thermophone 100 in the article includes a platinum strip 102, two terminal clamps 104 and a substrate 106. The platinum strip 102 and the two terminal clamps 104 are disposed on a surface of the substrate 106. The two terminal clamps 104 are located apart from each other, and are electrically connected to the platinum strip 102. The platinum strip 102 having a thickness of 0.7 micrometers. Frequency response range and sound pressure of sound wave are closely related to the heat capacity per unit area of the platinum strip 102. The higher the heat capacity per unit area, the narrower the frequency response range and the weaker the sound pressure. It's very difficult to produce an extremely thin metal strip (e.g., platinum strip). For example, the platinum strip 102 has a heat capacity per unit area higher than 2×10−4 J/cm2*K. The platinum strip 102 is difficult to generate ultrasonic wave. Further, the platinum strip 102 only generates sound wave in a gas medium such as air, although it could be very useful to produce sound waves in different mediums.
An ultrasonic acoustic device generally includes an ultrasonic transducer and a signal device. The ultrasonic transducer can be a resonance type ultrasonic transducer such as a vibration cell. The ultrasonic transducer converts an electrical signal into an ultrasonic sound. Ultrasonic transducers are usually complicated and include two piezoelectric ceramics, a cone, a shell and several conductive wires
What is needed, therefore, is to provide an ultrasonic acoustic device based on carbon nanotubes can have a simple structure, and able to propagate ultrasonic sound in more than one medium.